Rotary chucks with radially movable jaws for gripping work located at the rotary axis of the chuck are commonly used for machining operations. The jaws are moved through various types of internal mechanisms, for example, bell-crank type levers of which one arm engages the back of a movable jaw and the other cooperates with an axially movable lever actuator. Typically, the actuator is moved with a draw rod or tube connected through the back of the chuck and actuated by a hydraulic cylinder.
A universal chuck of lever-actuated pull-back type is disclosed in U.S. Pat. No. 3,396,982 owned by the assignee of the present application. This type of lever-actuated chuck provides high gripping forces and is therefore advantageously used where a heavy workpiece is to be held or where high speed or heavy cutting or both are employed. In the chucking operations for which this type of chuck is used, it is necessary that the workpiece be positioned with a reference surface of the workpiece against a transverse, i.e., radial, locating surface of the chuck. The chuck is constructed to pull the workpiece back after it is gripped to place the reference and locating surfaces into abutment. The prior art devices, however, do not provide any assurance or indication that the workpiece and the locating surface have in fact come into abutment.
Apart from proper location of the workpiece, adequate work-gripping force must be assured. As rotary machining speeds increase to reduce machining time, it becomes more important but also more difficult to maintain sufficient gripping force, because centrifugal force tends to open the jaws. By sensing the gripping force, a determination can be made that the workpiece is adequately gripped for the rotational and cutting forces involved.